Depends on the price point and why you are upgrading your 6800. The 8500GT benches about half what the 8600GT does in 3DMark03-06. The 8600GT is getting about 83-84% of the benchies across the board with 3DMark03-06 as the 8600GTS.

The main reason I'm even asking is, is it it worth it to upgrade for the DX10 capabilities and will I see a performance increase in existing games like BF2, Company of Heroes, or Counter-Strike: Source, or should I just wait until the 6800 becomes unusable for new games? Price point I'm looking at is about $150 - $175. Thanks for the help.

Well maybe someone with a bit more knowledge than me can chime in. But what I'd do is compare your 3DMark score with others around the net for your same card, so you have an idea what your card should be getting and what you are getting (there will be some variance) and then look at the 3dmark scores in that link I gave. 3DMark06 doesn't use DirectX 10 so you are safe with comparisons there.

I think you'll see even the 8500 will outperform your 6800, but I can't tell you how much because I don't know what it should be getting.

So basically yes I think the new DX10 cards will show you a performance increase in today's games. But personally I'd stick with the 6800 until you run into a game it just can't handle.